CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

14. The Formation of Species and Evolutionary Change

Another Piece of the Human Evolution Puzzle Unearthed

The newest fossil may reveal more information about our origins.

Just where would you expect to find a 47-million-year-old primate fossil? Africa, of course! But not this time. “Ida” (Darwinius masillae) was found in Messel Pit, a pit created by an oil shale mining operation in Germany in 1983, and not by a professional paleontologist, but an amateur collector. Fossil exploration began after the mining operation was completed and the pit was authorized to become a garbage dump. Ida was kept in a private collection for 25 years before she was acquired by the Natural History Museum of the University of Oslo for scientific study.

Ida is the most complete primate skeleton known in the fossil record. She has a complete skeleton, a soft body outline, and food in her digestive tract. Preliminary evidence reveals that she lived during the Eocene Epoch, after the extinction of dinosaurs and when primates split into two major groups: prosimians and anthropoids. The region was experiencing continental drift and just beginning to take on features we would recognize as Germany’s landscape today. During the Eocene, many modern plants and animals were evolving in a subtropical, jungle-like environment. Evolutionarily, Ida and her relatives are thought to have been the evolutionary base of the anthropoid branch that led to monkeys, apes, and humans.

Ida lacks traits found in lemurs, such as a grooming claw on the second toe of the foot, a fused row of teeth in the middle of her lower jaw (known as a toothcomb), and claws. Her more advanced traits include the presence of fingernails, forward-facing eyes (allowing her to have 3-D vision and the ability to judge distance), and teeth similar to those of monkeys. Ida also has a talus bone in her feet. This bone allows her entire weight to be transmitted to the foot, an important feature in bipedal animals.

• What role have fossils played in understanding species evolution?

• What factors are important to the formation of a new species?

• What do scientists know about the evolution of humans?

ü Background Check

Concepts you should already understand to get the most out of this chapter:

• The fundamentals of meiosis, genes, and alleles (chapter 10)

• Traits that make a population a species (chapter 12)

• The role natural selection plays in evolution (chapter 13)

14.1. Evolutionary Patterns at the Species Level

Chapter 13 focused on the concept of microevolution—that is, minor differences in allele frequency between populations of the same species, as when genetic differences between subspecies are examined. This chapter focuses on macroevolution, the major differences that have occurred over long periods that have resulted in so much genetic change that new kinds of species are produced. Furthermore, the present situation is not the end of the evolutionary process because evolution is still occurring today. Recall from chapter 12 that a species is a population of organisms whose members have the potential to interbreed naturally and to produce fertile offspring but do not interbreed with other groups. This inability of a species to generate fertile offspring after breeding with other more genetically different organisms is a key to understanding how a new species can originate from a preexisting ancestral species.

There are three key ideas within this definition. First, a species is a population of organisms. An individual is not a species. An individual can only be a member of a group that is a species. The human species, Homo sapiens, consists of over 7 billion individuals, whereas the endangered California condor species, Gymnogyps californianus, currently consists of about 322 individuals, 179 of which live in the wild.

Second, the definition takes into consideration the ability of individuals within the group to produce fertile offspring. Obviously, not every individual can be checked to see if it is capable of mating with any other individual that is similar to it, so we must make some judgment calls. Can most individuals within the population interbreed to produce fertile offspring? Although all humans are of the same species, some individuals are sterile and cannot reproduce. However, we don’t exclude them from the human species because of this. If they were not sterile, they would have the potential to interbreed. Although humans normally choose mating partners from their own local ethnic groups, humans from all parts of the world are potentially capable of interbreeding. This is known to be true because of the large number of instances of reproduction involving people of different ethnic backgrounds. The same is true for many other species that have local subpopulations but have a wide geographic distribution.

Third, the species concept also takes into account an organism’s evolutionary history. A species is a group of organisms that shares a common ancestor with other species, but is set off from those others by having newer, genetically unique traits.

So how do we know if two populations really do belong to the same species?

Gene Flow

One way to find out if two populations belong to the same species is to investigate gene flow. Gene flow is the movement of genes from one generation to the next as a result of reproduction or from one region to another by migration. Two or more populations that demonstrate gene flow among them constitute a single species. On the other hand, two or more populations that do not have gene flow through reproduction, when given the opportunity, are generally considered to be different species. Some examples will clarify this working definition.

Donkeys (also called asses) and horses are thought to be two different species, even though they can be mated and produce offspring, called mules (figure 14.1). Because mules are nearly always sterile and do not produce offspring, this is not considered to be gene flow, so donkeys and horses are considered separate species. Similarly, lions and tigers can be mated in zoos to produce offspring. However, this does not happen in nature, so gene flow does not occur naturally; thus, they are also considered two separate species.

FIGURE 14.1. Hybrid Sterility

Even though they do not do so in nature, (a) horses (Equus caballus) and (b) donkeys (Equus asinus) can be mated. The offspring produced by mating a female horse with a male donkey is called a (c) mule (Equus asinus x caballus) and is sterile. Because all mules are sterile, the horse and the donkey are considered to be of different species.

Genetic Similarity

Another way to find out if two organisms belong to different species is to determine their degree of genetic similarity. Advances in molecular genetics have allowed scientists to examine the sequence of bases in the genes present in individuals from a variety of populations. Those that have a great deal of similarity in their nitrogenous base sequences are assumed to have resulted from populations that have exchanged genes through sexual reproduction in the recent past. If there are significant differences, the two populations have probably not exchanged genes recently and are more likely to be members of separate species. There is no universal rule that states the smallest allowable genetic difference between two species. For example, genetic similarity between two South American field mice, Akodon dolores and A. molinae, have been analyzed to determine if they are, in fact, members of the same species located in different geographic regions. Experts examining the genetic differences concluded that they are the same species but different geographic subspecies. The interpretation of the results obtained by examining genetic differences still requires the judgment of experts. Although this technique is probably not the ultimate method to settle every dispute related to the identification of species, it is an important tool.